The present invention relates to an image forming apparatus having a printer head incorporating a liquid crystal array in which micro-shutters are arranged into an N.times.M matrix array together with a light source, and a driving method therefor.
There have been well known in the art liquid crystal display devices and liquid crystal shutter arrays of the type in which a number (n) of scanning electrodes and a number (m) of signal electrodes are arranged into a matrix array and a large number of picture elements or pixels and shutter apertures are formed by a liquid crystal which is a capacitance type load element. A method for driving a liquid crystal element is such that an address signal is sequentially, periodically and selectively applied to the scanning electrodes while predetermined data or information signals are applied selectively in time series or serially in synchronism with address signals to the signal electrodes. According to this driving system, as the number of time-divisions increases, the ratio V.sub.ON (ON signal)/V.sub.OFF (OFF signal) approaches unity as indicated by the following equation: ##EQU1## where 1/N: duty ratio;
1/a: bias ratio; and PA1 V.sub.0 : voltage applied.
Accordingly, the shutter performance of a liquid crystal element constituting a picture element is degraded. Especially, in the case of a liquid crystal shutter array, an optical signal with a satisfactory S/N ratio cannot be obtained so that, when such driving system as described above is used in a printer head of an electrophotographic printer, there arises a problem that a high-quality image cannot be formed.
More specifically, an optimum driving condition is 1/1 duty, in other words, a static drive. In this case, it is required that each picture element is controlled by a driver circuit. For instance, in case of the liquid crystal shutter array capable of generating light spots at a density of 16 dots per millimeter in the width direction of an A-4 size (Japanese Industrial Standards: 210.times.297 mm), 3360 drive circuits are required and 105 IC (integrated circuit devices) are required when 32 driver circuits are made on each IC. As a result, the static drive is not adapted to drive liquid crystal shutter arrays having a high density of picture elements or shutter apertures.
As described above, an optical signal generator which is used to impart optical signals into a photosensitive drum (an image bearing member) of an electrophotographic printer comprises a liquid crystal shutter array and a light source and, in order to obtain a high-quality print or image, a high S/N ratio must be provided between the shutter ON state and the shutter OFF state. In general, an S/N ratio higher than 5 is demanded. Meanwhile, a high processing speed is demanded of electrophotographic printers in these days, but no liquid crystal shutter array capable of allowing such a high processing speed has been provided yet.